Because carbon fibers have minimal ductility and are brittle, they are prone to fuzzing caused by mechanical friction and the like, and furthermore because they also display poor wetting characteristics relative to matrix resins, when carbon fibers are used as reinforcing materials for reinforced composite materials, the excellent properties offered by the carbon fibers as a reinforcing material are unable to be manifested satisfactorily.
In order to improve these defects, carbon fibers are conventionally subjected to a sizing treatment with a sizing agent, and various compounds continue to be tested as sizing agents.
For example, Japanese Unexamined Patent Application, First Publication No. Sho 61-28074 discloses an aqueous emulsion of a bisphenol type polyalkylene ether epoxy compound produced using a small quantity of an emulsifier, which is applied to carbon fiber. Japanese Unexamined Patent Application, First Publication No. Sho 50-59589 describes the application of a solvent based solution of a sizing agent comprising a polyglycidyl ether to carbon fiber.
Sizing agents comprising the types of bisphenol type polyalkylene ether epoxy compounds described above can be used as water based emulsions, which provides extremely favorable properties in terms of industrial processability and safety, and moreover because these compounds contain a polyalkylene ether chain within the molecule, they show good compatibility with water, and because they also contain an epoxy group, they exhibit excellent adhesion to matrix resins.
However, because they contain polyalkylene ether chains, sizing agents comprising these bisphenol type polyalkylene ether epoxy compounds produce carbon fiber reinforced composite materials for which shearing characteristics is unsatisfactory. Furthermore, the heat resistance of these sizing agents is low, and it is known, for example, that even a carbon fiber reinforced composite material obtained by combining carbon fiber treated with one of these sizing agents, and a 180° C. cured epoxy resin does not display expected heat resistance performance. Furthermore, in those cases in which a non-ionic surfactant is used as an emulsifier, the non-ionic surfactant will often comprise a polyethylene oxide, and this can cause further deterioration in the heat resistance of the carbon fiber reinforced composite material.
On the other hand, the latter type of sizing agents comprising a polyglycidyl ether utilize a solvent based solution of the compound as the sizing solution, and consequently these sizing agents are inferior to the former water based sizing agents in terms of industrial processability and safety within the sizing process. In particular, sizing agents comprising a bisphenol A type glycidyl ether exhibit poor wettability with the matrix resin if absolutely no emulsifier is used, and are unable to fabricate a favorable carbon fiber reinforced composite material.